Abstract: The internal friction of <111> tilt and twist grain boundaries in pure Al bicrystals with different misorientations was studied. The measured activation parameters of twist grain boundaries are different from those of tilt grain boundaries, which was explained by the difference of grain boundaries microstructure. With the help of a coupling model, the basic relaxation processes of different types of grain boundaries were suggested. In addition, when the misorientation angles of both tilt and twist grain boundaries are near to 60°, the grain boundarie internal friction peak does not appear, indicating that both coherent and incoherent near to 60° grain boundaries are highly resistant to stressinduced motion.